In many areas of production, it is vital that prescribed specifications of individual workpieces that will be assembled to form a larger product be observed. This necessitates that measurements and tests on workpieces are carried out, i.e., that they are examined. This is true even where a multiplicity of different workpieces must be examined such as during the manufacturing of very small batches or of highly complex apparatuses containing a large number of different elements, and often the examination must be performed quickly.
One possible examination method for typical measurement and test tasks is computed tomography (CT). CT images of workpieces permit a comparison between an actual shape thereof and a desired shape thereof and possibly also grant the ability to detect internal defects of a workpiece, such as shrinking holes inside the body and the like. However, as CT scanners are expensive, it is desirable to, as much as possible, reduce the time required to take an image of a workpiece using a CT scanner, in particular during manufacturing.
It has been suggested to transport workpieces on a carrier into the examination space of industrial CT scanners. Such carriers can be arranged as pallet systems or pallet stacks that are placed on a movable table inside the examination space. It may be possible in this case to arrange a plurality of parts next to one another or on top of one another, move them together into the industrial CT scanner, and finally successively prepare CT images of the individual workpieces inside the machine by moving a table, e.g., in the z-axis.
Although changeover times are reduced in this manner and although it becomes possible to examine different workpieces in quick succession using a CT scanner, not only must it be ensured that the examination results relating to the workpieces to be examined become available in short order, but it must also be ensured that the examination results are accurate enough and that they are correctly assigned.
This is important in particular where the production parameters of a current production process might need to be adapted on the basis of a measurement or test result.
It has already been suggested to identify the individual pallets of a pallet stack and different workpiece positions on a palette by way of suitable codes. Optically readable line codes or matrix codes, e.g., barcodes or QR codes, have already been used for this purpose. RFID chips that are readable by way of radio waves and the like have also already been used.
Another suggestion in the past has been to provide X-ray recordings and the like with blocks of varying density which together are intended to code characters and are photoelectrically readable from X-ray images, cf. JP 0 200 3881 A.
Another suggestion has been to generate a machine-readable mark from a multiplicity of labels arranged one on top of another, cf. U.S. Pat. No. 6,899,275 B2. One complete symbol can thus be coded in each layer or be divided into fragments that are distributed over a plurality of layers. Said document also discusses reading the machine-readable mark using ultrasound or X-ray techniques, among others.
EP 2 587 450 A1 discloses the use of positioning marks to be used to scale, rotate, dewarp, etc. X-ray recordings or tomographic recordings. It has been suggested to use marks in non-symmetrical form to reduce rotation errors and to provide clearly discernible shapes in an image. It has been suggested that a material that has been adapted to the X-rays of the corresponding energy is used in a size that is adapted to the image field, for example a diameter of 100 μm to 0.8 mm.